Method of image sticking prevention and recovery treatment for ferroelectric liquid crystal device

ABSTRACT

Disclosed is a method of treatment for preventing the occurrence of image sticking and recovering from image sticking in a ferroelectric liquid crystal device. By applying a low-frequency AC voltage to the ferroelectric liquid crystal device, multiple domains are formed in a pixel, domain boundaries are caused to flow, and fine segmentation of domain regions are induced. As the low-frequency AC voltage, a voltage having a frequency of 10 to 100 Hz and an amplitude of ±1.5 to ±3.0 V is applied, for example, for 30 seconds or more. The low-frequency AC voltage is applied to the liquid crystal device immediately before stopping liquid crystal driving or immediately after starting liquid crystal driving. Further, the low-frequency AC voltage is applied when the occurrence of image sticking in the liquid crystal device is detected.

TECHNICAL FIELD

The present invention relates to a method of treatment for improving thequality of a ferroelectric liquid crystal device. More particularly, theinvention relates to a method of treatment for preventing image stickingthat occurs when a liquid crystal is held in one of its stable statesfor a long period of time, and for recovering the liquid crystal fromimage sticking if image sticking occurs at all.

BACKGROUND ART

Liquid crystal devices are used not only for display devices but alsofor spatial light modulators such as optical shutters.

Ferroelectric liquid crystals have two stable states, hereinafter calledthe one stable state and the other stable state. In a liquid crystaldevice using a ferroelectric liquid crystal, an image is displayed, orthe device is operated as an optical shutter, by switching each pixelfrom the one stable state to the other stable state.

Ferroelectric liquid crystals have the property that when an appliedvoltage is removed, the immediately previous state, i.e., the one or theother of the stable states, is retained (this property is hereinaftercalled the “memory property”). Accordingly, the ferroelectric liquidcrystal, once set in a stable state, remains in a stable state until avoltage greater than the threshold is applied for a certain length oftime.

Further, since ferroelectric liquid crystal molecules each possessspontaneous polarization, when the ferroelectric liquid crystalmolecules in a pixel are in one stable state, an electric field pointingin one direction occurs in the liquid crystal layer of the pixel.

When one of the stable states is maintained for a long time, forexample, a few milliseconds or a few seconds, ionic impurities in theliquid crystal layer agglomerate by being attracted in the direction ofthe electric field. This disrupts the electric symmetry in the liquidcrystal layer. As a result, when the liquid crystal is switched from onestable state to the other stable state, a portion of the liquid crystalreturns to that one stable state and contrast degrades. This phenomenonis called the “image sticking phenomenon”. This phenomenon is a factorthat works to degrade the quality of the liquid crystal device. Forexample, when the ferroelectric liquid crystal device is used as adisplay device, if the image sticking phenomenon occurs, the previouslydisplayed image remains visible, degrading the display quality.

To prevent such image sticking, it is imperative to avoid setting allthe ferroelectric liquid crystal in the pixel in the same state whenleaving the ferroelectric liquid crystal in the non-driven state, i.e.,in one of the stable states, for a long period of time. That is, theelectric field formed in the liquid crystal layer by the spontaneouspolarization must be canceled by having the two stable states existsimultaneously.

Japanese Patent Unexamined Publication No. 2-225592

discloses a technique which involves adding an impurity ion adsorbingmaterial to the liquid crystal in order to prevent the occurrence ofimage sticking. However, it is not always possible to adsorb all theimpurity ions.

Further, Japanese Patent Unexamined Publication No. 2-165122 discloses atechnique which periodically applies positive and negative electricfields during the non-driven period to switch the direction of theliquid crystal alignment from one direction to another. This method,however, not only requires a complex circuit configuration, but does notfunction property if, for some reason, the power is cut off.

On the other hand, once image sticking has occurred, the liquid crystaldoes not recover by itself if the liquid crystal is driven in the usualway; therefore, a treatment for recovery becomes necessary. One knownmethod of recovery treatment involves the heat treatment hereinafterdescribed. The temperature of the liquid crystal is first raised to thetemperature of the isotropic phase, and then the temperature isgradually lowered to the temperature of the nematic phase; this causesthe long axes of the molecules to align in the same direction. When thetemperature is further lowered to the temperature of the smectic Aphase, not only the molecular long axes but also the molecular centersof mass align. When the temperature is further lowered into the smecticC phase, the molecules are realigned in the original state. As a result,the image sticking is removed, and the quality of the liquid crystaldevice is thus recovered. This series of heat treating operations iscalled the isotropic treatment.

A ferroelectric liquid crystal device designed to perform the isotropictreatment is equipped with a plate-like heating element for heating anda temperature sensing element for sensing the temperature of the liquidcrystal so that the temperature can be raised to the temperature of theisotropic phase.

The prior art isotropic treatment, however, requires heating the wholepanel to heat all the liquid crystal elements to the temperature of theisotropic phase and then gradually cooling it to the operatingtemperature. This clearly reduces time efficiency and increases powerconsumption.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a simple and reliablemethod of treatment for image sticking prevention to prevent the imagesticking that occurs when the ferroelectric liquid crystal in aferroelectric liquid crystal device is held in one of its stable statesfor a long time. Earlier, the length of time to hold the liquid crystalin one of the stable states was described as being a few milliseconds ora few seconds. In contrast, in the present invention, image sticking canbe prevented even when the liquid crystal has been held in one of thestable states for a few hours or tens of hours.

Another object of the present invention is to provide a simple andreliable method of recovery treatment for recovering the ferroelectricliquid crystal device from image sticking if image sticking occurs atall in the ferroelectric liquid crystal device.

To achieve the above objects, according to the treatment of the presentinvention, a low-frequency AC voltage is applied to the ferroelectricliquid crystal device, thereby forming multiple domains in a pixel,causing domain boundaries to flow, and inducing fine segmentation ofdomain regions.

(Operation)

In operation, the treatment for preventing the occurrence of imagesticking is performed in the following manner.

In the ferroelectric liquid crystal device, when leaving the liquidcrystal in one of its stable states for a long time, an AC voltage,whose frequency is distinctly lower than that of the usual liquidcrystal driving waveform and whose amplitude does not exceed thethreshold of the liquid crystal, is applied in advance to the liquidcrystal device. For example, an AC voltage having a frequency of 10 to100 Hz and an amplitude of ±1.5 to ±3.0 V is applied for about oneminute. As a result, domains consisting of two stable states are formedwithin a pixel, and the domains are repeatedly segmented with the domainboundaries flowing in an irregular manner, eventually resulting in theformation of finely dispersed multiple domains.

In the ferroelectric liquid crystal device treated as described above,when the liquid crystal was driven in the usual way after leaving it inone of its stable states for a long time, image sticking did not occur.

On the other hand, the treatment for recovering the liquid crystal fromimage sticking is performed in the following manner.

In the ferroelectric liquid crystal where image sticking has occurredafter being left in one of its stable states for a long time, an ACvoltage, whose frequency is distinctly lower than that of the usualliquid crystal driving waveform and whose amplitude does not exceed thethreshold of the liquid crystal, is applied in the same manner asearlier described. For example, an AC voltage having a frequency of 10to 100 Hz and an amplitude of ±1.5 to ±3.0 V is applied. As a result,domains consisting of two stable states are formed within a pixel. Then,the domains are repeatedly segmented with the domain boundaries flowingin an irregular manner, eventually resulting in the formation of finelydispersed multiple domains.

In the ferroelectric liquid crystal device treated as described above,image sticking did not occur in the subsequent liquid crystal driving,and it was observed that the electric symmetry in the liquid crystalcell had been nearly perfectly restored.

ADVANTAGEOUS EFFECT OF THE INVENTION

Using the treatment method of the present invention, the image stickingphenomenon that would occur in the ferroelectric liquid crystal when theliquid crystal was held in one of its stable states can be prevented ina simple and reliable manner, by applying the low-frequency AC voltage.

Further, by using the treatment method of the present invention,recovery from image sticking can be accomplished in a short period oftime in a simple manner, by applying the low-frequency AC voltage andwithout specifically requiring the provision of heating means or thelike. Accordingly, by incorporating the low-frequency AC voltageapplication means into the liquid crystal device together with imagesticking detection means, image sticking of the liquid crystal devicecan be prevented and, further, recovery from image sticking can beaccomplished. As a result, the liquid crystal device of high quality canalways be obtained without depending on the history of the liquidcrystal state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the arrangement of a ferroelectric liquidcrystal cell and polarizers.

FIG. 2 is a diagram showing how the light transmittance of theferroelectric liquid crystal cell varies with an applied voltage.

FIG. 3 is a diagram showing the structure of the ferroelectric liquidcrystal cell used in the embodiments of the present invention.

FIG. 4 is a diagram showing a voltage waveform applied to a liquidcrystal and the variation in the amount of light transmission indicatingthe occurrence of image sticking.

FIG. 5 is a diagram showing the applied voltage waveform and thevariation in the amount of light transmission according to theembodiment of the image sticking prevention treatment method of thepresent invention.

FIG. 6 is a diagram showing how the state of the ferroelectric liquidcrystal device and the domain structure in a pixel change when the imagesticking prevention and recovery treatment according to the embodimentof the present invention is performed.

FIG. 7 is a diagram showing the voltage waveform applied to the liquidcrystal and the variation in the amount of light transmission indicatingthe occurrence of image sticking.

FIG. 8 is a diagram showing the applied voltage waveform and thevariation in the amount of light transmission according to theembodiment of the image sticking recovery treatment method of thepresent invention.

FIG. 9 is a block diagram of a driving circuit for the ferroelectricliquid crystal device, which is used to implement the treatment methodof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram showing the arrangement of polarizers whenferroelectric liquid crystal is used as a liquid crystal cell. Betweenthe polarizers 1 a and 1 b arranged in a crossed Nicol configuration isplaced the liquid crystal cell 2 in such a manner that the long axisdirection of liquid crystal molecules when the molecules are in a firststable state or in a second stable state is oriented substantiallyparallel to either the polarization axis, a, of the polarizer 1 a or thepolarization axis, b, of the polarizer 1 b.

When voltage is applied across the thus arranged liquid crystal cell,its light transmittance varies with the applied voltage, describing aloop as plotted in the graph of FIG. 2. The voltage value at which thelight transmittance begins to change when the applied voltage isdecreased is denoted by V1, and the voltage value at which the lighttransmittance reaches saturation is denoted by V2; on the other hand,the voltage value at which the light transmittance begins to drop whenthe applied voltage is increased into the region of the oppositepolarity is denoted by V3, and the voltage value at and beyond which thelight transmittance does not drop further is denoted by V4. As shown inFIG. 2, the first stable state is selected when the value of the appliedvoltage is greater than the threshold of the ferroelectric liquidcrystal molecules. When the voltage of the opposite polarity greaterthan the threshold of the ferroelectric liquid crystal molecules isapplied, the second stable state is selected.

When the polarizers are arranged as shown in FIG. 1, a black displaystate (non-transmission state) can be produced in the first stable stateand a white display state (transmission state) in the second stablestate. The arrangement of the polarizers can be changed so that thewhite display state (transmission state) is produced in the first stablestate and the black display state (non-transmission state) in the secondstable state. The description hereinafter given, however, assumes thatthe polarizers are arranged so as to produce the black display state(non-transmission state) in the first stable state and the white displaystate (transmission state) in the second stable state.

FIG. 3 is a cross-sectional view showing the cell structure of aferroelectric liquid crystal device used in the embodiments of thepresent invention. The ferroelectric liquid crystal cell used in theembodiments comprises: a ferroelectric liquid crystal layer with athickness of about 1.7 μm; a pair of glass substrates 12 a and 12 bsandwiching the ferroelectric liquid crystal layer 11; a spacer member13 for maintaining a gap; and a sealing member 14 for sealing theferroelectric liquid crystal layer 11 against the outside environment.Scanning electrodes 15a and signal electrodes 15b are formed on theopposing surfaces of the glass substrates 12 a and 12 b, respectively.Insulating films 18 a and 18 b of tantalum pentoxide or the like forpreventing short-circuiting are formed over the electrodes 15 a and 15b, and obliquely evaporated inorganic alignment films 16 a and 16 b ofsilicon oxide or the like for aligning ferroelectric liquid crystalmolecules are formed on top of them. On the outside surfaces of theglass substrates 12 a and 12 b are arranged polarizers 17 a and 17 bwith the polarization axis of the polarizer 17 a oriented parallel tothe long axis direction of the ferroelectric liquid crystal moleculeswhen the molecules are in one of the two stable states, and with thepolarization axis of the polarizer 17 b oriented at right angles to thepolarization axis of the polarizer 17 a.

FIG. 4 show graphs depicting how the driving waveform and the amount oflight transmission change in the ferroelectric liquid crystal devicewhen the image sticking prevention treatment of the present invention isnot applied. FIG. 4(a) shows the voltage waveform applied to the liquidcrystal, and FIGS. 4(b) and 4(c) each show the variation with time ofthe amount of light transmission when the voltage of FIG. 4(a) isapplied.

In period A in FIG. 4, the liquid crystal is driven in the usual way. Attime {circle around (1)}, driving power is turned off to stop the liquidcrystal driving. In the next period B, the liquid crystal is not driven;then, to operate the liquid crystal device again, for example, 24 hourslater, the power is turned on at time {circle around (2)} to startdriving the liquid crystal. At time {circle around (3)} after period Chas elapsed from time {circle around (2)}, a voltage pulse P greaterthan the threshold of the liquid crystal is applied as the usual drivingpulse, causing the liquid crystal to switch from one stable state to theother stable state or vice versa, and the state is maintained duringperiod D.

FIG. 4(b) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in bright state in periodA. In FIG. 4(b), though the driving power is turned off at time {circlearound (1)}, the liquid crystal is held in the bright state, forexample, for 24 hours (period B indicated by dashed line). Next, whenthe power is turned on at time {circle around (2)} to start driving theliquid crystal, the liquid crystal state remains unchanged, i.e., heldin the bright state. Accordingly, if this liquid crystal device is usedas a display device, when the power is turned on at time {circle around(2)}, the liquid crystal device is in the bright state (period C). Next,when the pulse P is applied at time {circle around (3)}, the liquidcrystal switches from one stable state to the other stable state. For ashort period immediately following the switching, the liquid crystal isheld in the dark state with its state having switched to the otherstable state, and a high contrast is obtained. However, since the liquidcrystal has been held in the bright state for a long period of time, theliquid crystal state drifts in the direction of the bright state as thetime elapses. As a result, the amount of light transmission changes, andthe contrast degrades. This phenomenon is the “image stickingphenomenon”.

FIG. 4(c) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the dark state in periodA. In FIG. 4(c), though the driving power is turned off at time {circlearound (1)}, the liquid crystal is held in the dark state (period Bindicated by dashed line). Next, when the power is turned on at time{circle around (2)} to start driving the liquid crystal, the liquidcrystal state remains unchanged, i.e., held in the dark state.Accordingly, if this liquid crystal device is used as a display device,when the power is turned on at time {circle around (2)}, the liquidcrystal device is in the dark state (period C). Next, when the pulse Pis applied at time {circle around (3)}, the liquid crystal switches fromone stable state to the other stable state. For a short periodimmediately following the switching, the liquid crystal is held in thebright state with its state having switched to the other stable state,and a high contrast is obtained. However, since the liquid crystal hasbeen held in the dark state for a long period of time, the liquidcrystal state drifts in the direction of the dark state as the timeelapses. As a result, the amount of light transmission changes, and thecontrast degrades.

FIG. 5 show graphs depicting how the driving waveform and the amount oflight transmission change in the ferroelectric liquid crystal devicewhen the image sticking prevention treatment of the present invention isapplied. FIG. 5(a) shows the voltage waveform applied to the liquidcrystal, and FIGS. 5(b) and 5(c) each show the variation with time ofthe amount of light transmission when the voltage of FIG. 5(a) isapplied.

In period A in FIG. 5, the liquid crystal is driven in the usual way.For the duration of period a starting at time @ and ending immediatelybefore turning off the driving power to stop the liquid crystal driving,a low-frequency AC voltage is applied to the liquid crystal. Forexample, a rectangular AC wave having a frequency of 60 Hz and anamplitude of ±2.5 V is applied for one minute. Then, at time {circlearound (1)}, the driving power is turned off. In the next period B, theliquid crystal is not driven; then, to operate the liquid crystal deviceagain, for example, 24 hours later, the power is turned on at time{circle around (2)} to start driving the liquid crystal. At time {circlearound (3)} after period C has elapsed from time {circle around (2)}, avoltage pulse P greater than the threshold of the liquid crystal isapplied as the usual driving pulse, causing the liquid crystal to switchfrom one stable state to the other stable state or vice versa, and thestate is maintained during period D.

FIG. 5(b) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the bright state inperiod A. In FIG. 5(b), when the low-frequency AC voltage is applied tothe liquid crystal at time @, the liquid crystal state fluctuates untilit settles at a state intermediate between the bright and dark states attime {circle around (1)} when the power is turned off. Though thedriving power is turned off at time {circle around (1)}, the liquidcrystal is held in the intermediate state (period B indicated by dashedline). Next, when the power is turned on at time {circle around (2)} tostart driving the liquid crystal, the liquid crystal state remainsunchanged, i.e., held in the intermediate state. Accordingly, if thisliquid crystal device is used as a display device, when the power isturned on at time {circle around (2)}, the liquid crystal device is inthe intermediate state (period C). Next, the pulse P is applied at time{circle around (3)} to set the liquid crystal in the dark state. In thiscase, no temporal variation was observed in the liquid crystal state,and contrast degradation did not occur. That is, the “image stickingphenomenon” was successfully prevented.

FIG. 5(c) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the dark state in periodA. In FIG. 5(c), when the low-frequency AC voltage is applied to theliquid crystal at time @, the liquid crystal state fluctuates until itsettles at a state intermediate between the bright and dark states attime {circle around (1)} when the power is turned off. Though thedriving power is turned off at time {circle around (1)}, the liquidcrystal is held in the intermediate state (period B indicated by dashedline). Next, when the power is turned on at time {circle around (2)} tostart driving the liquid crystal, the liquid crystal state remainsunchanged, i.e., held in the intermediate state. Accordingly, if thisliquid crystal device is used as a display device, when the power isturned on at time {circle around (2)}, the liquid crystal device is inthe intermediate state (period C). Next, the pulse P is applied at time{circle around (3)} to set the liquid crystal in the bright state. Inthis case also, no temporal variation was observed in the liquid crystalstate, and contrast degradation did not occur. That is, the “imagesticking phenomenon” was successfully prevented.

In the above embodiment, the low-frequency AC voltage is applied whenturning off the power to stop the liquid crystal driving; alternatively,the low-frequency AC voltage may be applied when turning on the power tostart liquid crystal driving, in which case also the “image sticking”phenomenon can be prevented.

FIG. 6 is a diagram showing how the state of the ferroelectric liquidcrystal device and the domain structure in a pixel change when the imagesticking prevention treatment of the embodiment shown in FIG. 5 isapplied. FIGS. 6(d), 6(e), and 6(f) are enlarged views of importantportions of FIGS. 6(a), 6(b), and 6(c), respectively.

As shown in FIG. 6(a), each pixel in the liquid crystal device is set ineither the dark state or the bright state. Each pixel in the dark stateforms a single domain consisting of one stable state as shown, forexample, in FIG. 6(d). The previously described image stickingphenomenon occurs in the ferroelectric liquid crystal device held inthis state for 24 hours.

When the low-frequency AC voltage is applied to the ferroelectric liquidcrystal device where image sticking has occurred, a domain consisting ofthe other stable state is formed and a multi-domain structure results,as shown in FIG. 6(e). The domains are then gradually segmented withtheir boundaries flowing in an irregular manner in response to thelow-frequency AC voltage, and eventually, a microscopic multi-domainstructure with the segmented domains uniformly dispersed is formed asshown in FIG. 6(f). In this process, the liquid crystal state isaveraged and set in the intermediate state, as shown in FIGS. 6(b) and6(c). The gentle flow of the domain regions observed in this process andthe application of the low-frequency AC voltage inducing the flow arethe essential requirements for the prevention of image sticking.

In the present embodiment, a rectangular AC wave having a frequency of60 Hz and an amplitude of ±2.5 V is applied as the low-frequency ACvoltage, but the same effect can be obtained if an AC voltage with afrequency in the range of 10 to 100 Hz and an amplitude in the range of±1.5 V to ±3 V is used. Further, in the above embodiment, the length oftime to apply the low-frequency AC voltage has been set to one minute,but it has been found a similar effect can be obtained as long as thelength of time is set to 30 seconds or longer. However, since the lengthof time to apply the low-frequency AC voltage differs depending on theliquid crystal device used, the duration of that time should beappropriately set for each individual liquid crystal device. The aboveembodiment has been described assuming that the time interval from thetime the power is turned off until the liquid crystal operation isresumed is 24 hours, but it will be recognized that if this timeinterval is longer or shorter than 24 hours, image sticking of theliquid crystal device can be prevented by using the treatment method ofthe present invention.

Further, gradually increasing the frequency and/or gradually decreasingthe amplitude of the low-frequency AC voltage after the application ofthe low-frequency AC voltage would be an effective method; by so doing,a more uniform domain dispersion can be obtained.

The above embodiment has been described by taking as an example the casewhen power is turned off to an electronic appliance equipped with theliquid crystal device. It will, however, be appreciated that the aboveembodiment can be applied not only at the time of power off but also atthe time that the driving of the electronic appliance equipped with theliquid crystal device is stopped, in which case also the same effect canbe obtained. The same effect can also be obtained if the aboveembodiment is applied when shipping the ferroelectric liquid crystaldevice from the factory.

FIG. 7 show graphs depicting how the driving waveform and the amount oflight transmission change in the ferroelectric liquid crystal devicewhen the image sticking recovery treatment of the present invention isnot applied. FIG. 7(a) shows the voltage waveform applied to the liquidcrystal, and FIGS. 7(b) and 7(c) each show the variation with time ofthe amount of light transmission when the voltage of FIG. 7(a) isapplied.

In period B in FIG. 7, the liquid crystal is driven; it is assumed herethat the liquid crystal has been held in one of the stable states for along period of time, for example, 24 hours. Then, at time {circle around(3)}, a voltage pulse P greater than the threshold of the liquid crystalis applied as a driving pulse, causing the liquid crystal to switch fromone stable state to the other stable state or vice versa, and the stateis maintained during period D.

FIG. 7(b) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the bright state inperiod B. In FIG. 7(b), when the pulse P is applied at time {circlearound (3)}, the liquid crystal switches from one stable state to theother stable state. For a short period immediately following theswitching, the liquid crystal is held in the dark state with its statehaving switched to the other stable state, and a high contrast isobtained. However, the liquid crystal state drifts in the direction ofthe bright state as the time elapses. As a result, the amount of lighttransmission changes, and the contrast degrades. That is, the “imagesticking phenomenon” occurs.

FIG. 7(c) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the dark state in periodB. In FIG. 7(c), when the pulse P is applied at time {circle around(3)}, the liquid crystal switches from one stable state to the otherstable state. For a short period immediately following the switching,the liquid crystal is held in the bright state with its state havingswitched to the other stable state, and a high contrast is obtained.However, the liquid crystal state drifts in the direction of the darkstate as the time elapses. That is, the image sticking phenomenonoccurs, i.e., the amount of light transmission changes and the contrastdegrades.

FIG. 8 show graphs depicting how the driving waveform and the amount oflight transmission change in the ferroelectric liquid crystal devicewhen the image sticking recovery treatment of the present invention isapplied. FIG. 8(a) shows the voltage waveform applied to the liquidcrystal, and FIGS. 8(b) and 8(c) each show the variation with time ofthe amount of light transmission when the voltage of FIG. 8(a) isapplied.

The liquid crystal is driven in period B in FIG. 8. When the occurrenceof image sticking is detected during the driving of the liquid crystal,a low-frequency AC voltage is applied to the liquid crystal for theduration of period a starting at time @ partway through the liquidcrystal driving period. For example, a rectangular AC wave having afrequency of 60 Hz and an amplitude of ±2.5 V is applied for one minute.Next, when a voltage pulse P greater than the threshold of the liquidcrystal is applied as a driving pulse at time {circle around (3)}, theliquid crystal switches from one stable state to the other stable stateor vice versa, and the state is maintained during period D.

FIG. 8(b) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the bright state inperiod B. When the low-frequency AC voltage is applied to the liquidcrystal at time @ in FIG. 8(b), the liquid crystal state fluctuatesuntil it settles at a state intermediate between the bright and darkstates at the end of the low-frequency AC voltage application. Next, thepulse P is applied at time {circle around (3)} to set the liquid crystalin the dark state. In this case, no temporal variation was observed inthe liquid crystal state, and contrast degradation did not occur. Thatis, the liquid crystal was successfully recovered from the “imagesticking phenomenon”.

FIG. 8(c) is a graph showing the variation of the amount of lighttransmission when the liquid crystal was set in the dark state in periodB. In FIG. 8(c), when the low-frequency AC voltage is applied to theliquid crystal at time @, the liquid crystal state fluctuates until itsettles at a state intermediate between the bright and dark states atthe end of the low-frequency AC voltage application. Next, the pulse Pis applied at time {circle around (3)} to set the liquid crystal in- thebright state. In this case also, no temporal variation was observed inthe liquid crystal state, and contrast degradation did not occur. Thatis, the electric symmetry in the ferroelectric liquid crystal cell wasrestored, and the liquid crystal was successfully recovered from the“image sticking phenomenon”.

In the above embodiment, the recovery treatment was performed afterdetecting the occurrence of image sticking, but the recovery treatmentmay be performed after the liquid crystal screen has changed apredetermined number of times.

During the image sticking recovery treatment of the embodiment shown inFIG. 8, the state of the ferroelectric liquid crystal device and thedomain structure in a pixel change with time in the same manner as thestructural changes explained with reference to FIG. 6. The low-frequencyAC voltage, the duration of application, etc. are the same as those usedin the embodiment shown in FIG. 5.

FIG. 9 is a block diagram of a driving circuit for the ferroelectricliquid crystal device, which is used to implement the treatment methodof the present invention.

A voltage waveform from a liquid crystal driving voltage waveformgenerating circuit 12 or a low-frequency AC voltage waveform generatingcircuit 13 is applied via a selector switch SW to the ferroelectricliquid crystal device 11. The voltage waveform applied from the liquidcrystal driving voltage waveform generating circuit 12 is for usualliquid crystal driving, while the voltage waveform applied from thelow-frequency AC voltage waveform generating circuit 13 is for imagesticking prevention and recovery according to the present invention. Inthe usual liquid crystal driving, the driving voltage is applied fromthe liquid crystal driving voltage waveform generating circuit 12 to theliquid crystal device 11, based on a control signal from a liquidcrystal driving control circuit 14. A power supply circuit 15 suppliespower for operating the ferroelectric liquid crystal device.

On the other hand, the liquid crystal driving control circuit 14 sendsout a timing signal ta defining the timing for the low-frequency ACvoltage waveform generating circuit 13 to apply the low-frequency ACvoltage waveform to the liquid crystal device 11 during the liquidcrystal driving. The power supply circuit 15 sends out a timing signaltb defining the timing for applying the low-frequency AC voltagewaveform to the liquid crystal device 11 when turning power on or off.Further, an external circuit 16 sends out a timing signal tc definingthe timing for applying the low-frequency AC voltage waveform to theliquid crystal device 11 at an arbitrary time. These timing signals aresupplied via an OR circuit 17 to the SW, and the SW is thrown to thelow-frequency AC voltage waveform generating circuit 13 side. Then, thelow-frequency AC voltage waveform is applied to the liquid crystaldevice 11 for a predetermined length of time, for example, one minute.

The timing signal ta from the liquid crystal driving control circuit 14occurs when the occurrence of image sticking is detected during thedriving of the liquid crystal. Further, the timing signal ta occursafter the liquid crystal screen has changed a predetermined number oftimes, for example, tens of thousands of times. With this timing signal,the SW is thrown to apply the low-frequency AC voltage waveform as shownin FIG. 8, thereby recovering the liquid crystal from image sticking andpreventing the occurrence of image sticking.

The timing signal tb from the power supply circuit 15 occurs immediatelybefore the power is turned off when a power off operation is performed.With this timing signal, the SW is thrown to apply the low-frequency ACvoltage waveform as shown in FIG. 5, thereby preventing the occurrenceof image sticking. Further, when a power on operation is performed, thepower supply circuit 15 sends the timing signal tb, and the SW is thrownto apply the low-frequency AC voltage waveform, thereby preventing imagesticking from occurring in the subsequent liquid crystal driving.

The timing signal tc is sent from the external circuit 16 at anarbitrary time, for example, during the period when the power is off andthe liquid crystal device is not being driven. With this timing signal,the SW is thrown to apply the low-frequency AC voltage waveform, therebypreventing image sticking from occurring when the liquid crystal drivingis resumed.

The timing signal tc is also sent from the external circuit 16 at anarbitrary time during the liquid crystal driving. With this timingsignal, the SW is thrown to apply the low-frequency AC voltage waveformto prevent image sticking from occurring thereafter.

The external circuit 16 may be constructed, for example, from aswitching circuit which is operated at an arbitrary time to turn on theswitch and send out the timing signal tc. Further, the switch may beturned on automatically at prescribed intervals of time to send out thetiming signal tc.

What is claimed is:
 1. A method of image sticking prevention andrecovery treatment for a ferroelectric liquid crystal device having aferroelectric liquid crystal layer and electrodes to which a firstvoltage is applied for driving the liquid crystal device, wherein alow-frequency AC voltage, distinct from the first driving voltage usedto drive the liquid crystal device, is applied to the electrodes forforming multiple domains in which bright display regions and darkdisplay regions are intermingled in a pixel in said ferroelectric liquiddevice, causing domain boundaries to flow, and inducing finesegmentation of domain regions, said low-frequency AC voltage having afrequency of 10 to 100 Hz and an amplitude of ±1.5 to ±3.0 V and appliedto said ferroelectric liquid crystal device for a predetermined period.2. A method of image sticking prevention and recovery treatment for aferroelectric liquid crystal device as claimed in claim 1, wherein afterapplying said low-frequency AC voltage for said predetermined period,either the frequency of said low-frequency AC voltage is graduallyincreased or the amplitude thereof is gradually decreased.
 3. A methodof image sticking prevention and recovery treatment for a ferroelectricliquid crystal device as claimed in claim 1 or 2, wherein saidpredetermined period is not shorter than 30 seconds.
 4. A method ofimage sticking prevention and recovery treatment for a ferroelectricliquid crystal device as claimed in claim 1, wherein said low-frequencyAC voltage is applied immediately before driving of said ferroelectricliquid crystal device is stopped.
 5. A method of image stickingprevention and recovery treatment for a ferroelectric liquid crystaldevice as in claim 1, wherein said low-frequency AC voltage is appliedimmediately after driving of said ferroelectric liquid crystal device isstarted.
 6. A method of image sticking prevention and recovery treatmentfor a ferroelectric liquid crystal device as in claim 1, wherein saidlow-frequency AC voltage is applied prior to shipping said ferroelectricliquid crystal device from factory.
 7. A method of image stickingprevention and recovery treatment for a ferroelectric liquid crystaldevice as in claim 1, wherein said low-frequency AC voltage is appliedwhen the occurrence of image sticking is detected during driving of saidferroelectric liquid crystal device.
 8. A method of image stickingprevention and recovery treatment for a ferroelectric liquid crystaldevice as in claim 1, wherein said low-frequency AC voltage is appliedwhen a liquid crystal screen has changed a predetermined number of timesduring driving of said ferroelectric liquid crystal device.
 9. A methodof image sticking prevention and recovery treatment for a ferroelectricliquid crystal device as in claim 1, wherein said low-frequency ACvoltage is applied at an arbitrary time when said ferroelectric liquidcrystal device is not being driven.
 10. A method of image stickingprevention and recovery treatment for a ferroelectric liquid crystaldevice as in claim 1, wherein said low-frequency AC voltage is appliedat an arbitrary time when said ferroelectric liquid crystal device isbeing driven.
 11. A ferroelectric liquid crystal device having at leastone pixel, comprising a first voltage waveform generating circuit fordriving the ferroelectric liquid crystal device to set the at least onepixel in a bright or dark state; a second voltage waveform generatingcircuit for generating a low-frequency AC voltage waveform for formingmultiple domains in which bright display regions and dark displayregions are intermingled in the at least one pixel in the ferroelectricliquid crystal device, causing domain boundaries to flow, and inducingfine segmentation of domain regions; and a switch for applying whenactuated the low-frequency AC voltage waveform to the at least onepixel, and wherein the low-frequency AC voltage waveform is an ACvoltage with a frequency of 10 to 100 Hz and an amplitude of ±1.5 to±3.0 V, and is applied to the at least one pixel for at least 30 secondsby the switch when actuated.
 12. A ferroelectric liquid crystal deviceaccording to claim 11, further comprising a timing signal source foractuating the switch when an image sticking phenomenon occurs during thedriving of the ferroelectric liquid crystal device or after a liquidcrystal display containing the at least one pixel changes apredetermined number of times.
 13. A ferroelectric liquid crystal deviceaccording to claim 11, further comprising a power supply circuit forgenerating a timing signal to actuate the switch immediately before thefirst voltage waveform generating circuit is turned OFF or when thefirst voltage waveform generating circuit is turned ON.
 14. Aferroelectric liquid crystal device according to claim 11, furthercomprising a circuit for generating a timing signal to actuate theswitch at arbitrary times when the ferroelectric liquid crystal deviceis being driven by the first voltage waveform generating circuit.
 15. Aferroelectric liquid crystal device according to any one of claim 11-14,wherein the at least one pixel comprises a plurality of pixels arrangedin a matrix.
 16. A ferroelectric liquid crystal device having at leastone pixel, comprising a first voltage waveform generating circuit fordriving the ferroelectric liquid crystal device to set the at least onepixel in a bright or dark state; a second voltage waveform generatingcircuit for generating a low-frequency AC voltage waveform for forming adomain consisting of one stable state and a domain consisting of anotherstable state in the at least one pixel in said ferroelectric liquidcrystal device, resulting in a multi-domain structure, causing domainboundaries to flow, and inducing fine segmentation of domain regions;wherein the low-frequency AC voltage waveform is an AC voltage with afrequency of 10 to 100 Hz and an amplitude of ±1.5 to ±3.0 V; and aswitch for applying when actuated the low-frequency AC voltage waveformto the at least one pixel for a predetermined period.
 17. Aferroelectric liquid crystal device according to claim 16 wherein the atleast one pixel comprises a plurality of pixels arranged in a matrix.